Unusual 2:3:2 complex of DABCO mono-betaine with HCl and H2O studied by X-ray diffraction, DFT calculations and spectroscopic methods

DABCO mono-betaine (1,4-diazabicyclo[2.2.2]octane-1-acetate) forms a complex with HCl and water in the ratio 2:3:2. The crystals are triclinic, space group P  1¯. Two non-equivalent molecules of protonated DABCO mono-betaines form a homoconjugated cation through the short and asymmetric O·H·O hydrogen bond of 2.470(3) Å. Two chloride anions are involved in the N–H···Cl hydrogen bonds of 3.049(2) and 3.023(3) Å. The third chloride anion is solvated by the water molecules. The molecules of bis(1,4-diazoniumbicyclo[2.2.2]octane-1-acetate) trihydrochloride dihydrate are linked into comb-like chains by the hydrogen bond formed between water molecules of the neighboring complexes; the O(W)–H···O(W) distance is 2.920(5) Å. FTIR spectrum shows several broad bands attributed to the νO–H, νN–H and νO·H·O vibrations. The structure of three bis(1,4-diazoniumbicyclo[2.2.2]octane-1-acetate) hydrochlorides are optimized at the B3LYP/6-31G(d,p) level of theory and a theoretical IR spectrum is calculated. The values of pKa of DABCO, DABCO mono- and di-betaines are determined by the potentiometric titration of their hydrohalides.